llvmpipe: Fix PIPE_FORMAT_Z32_FLOAT_S8X24_UINT handling for big-endian.

llvmpipe: Fix PIPE_FORMAT_Z32_FLOAT_S8X24_UINT handling for big-endian.

llvmpipe treats PIPE_FORMAT_Z32_FLOAT_S8X24_UINT as a bit of a special case,
handling it as two 32-bit pieces rather than a single 64-bit block:

   /* 64bit d/s format is special already extracted 32 bits */
   total_bits = format_desc->block.bits > 32 ? 32 : format_desc->block.bits;

The format_desc describes the whole 64-bit block, so the z shift
will be 32 for big-endian.  But since we're accessing the z channel
as a 32-bit value rather than a 64-bit value, we need to mask the shift
with 31.

Signed-off-by: Richard Sandiford <rsandifo@linux.vnet.ibm.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>